1. Field of the Invention
The present invention relates to a method and apparatus for operating a magnetic clutch. More specifically, the present invention relates to a control circuit for effecting fast and positive uncoupling of a magnetic clutch.
2. Description of the Prior Art
Heretofore the productivity of machine tools has been greatly influenced by the rotational speed at which the machine tool can operate.
More particularly, punch presses, shears and other single cycle operating devices utilizing a mechanical pin clutch in combination with friction brakes have their cyclic rate determined by the rotational speed of a prime mover, the time delays in engaging a ram clutch pin to the prime mover and the time it takes to disengage the prime mover and stop the ram. Here the rotational rate or speed is limited primarily by engagement shock inherent with the mechanical clutch.
With the developement of sophisticated electronic and electro-optical control circuits and in particular electro-optical single and dual axis correctors to automatically sense and position a registration mark in a machine for punching, drilling, shearing or performing whatever machining requirements are desired, it becomes advantageous to maximize the productivity of the machine tool by shortening or eliminating all unnecessary system delays.
In such sophisticated systems, electrical clutches and brakes have been used instead of mechanical clutches. Such electrical clutches and brakes rely upon magnetically activated ferromagnetic members to couple a mechanical power transmission element to a driven element.
In a typical electromagnetic clutch, a revolving ferromagnetic disc is coupled by magnetic attraction to another ferromagnetic disc on a shaft to cause it to rotate. Usually the electromagnet producing the coupling magnetic flux is stationary and the flux passes through an air gap through a rotation rotor and into a coupling disc.
Several time delays are associated with an electromagnetic coupling device and the operation thereof. These delays can be summarized as follows:
1. The delay of flux buildup upon application of power to a clutch coil. PA0 2. The delay of mechanically moving the coupling disc members for engagement after flux buildup. PA0 3. The delay of flux decay upon removing power from the clutch coil. PA0 4. The drop out of the coupling disc members after the flux decay.
The first delay is determied by the L/R charging circuit characteristics where L is the inductance and R is the resistance of the clutch magnetizing coil. This delay can be minimized by providing conventional hi-impedance, hi-voltage driving circuits.
The second delay is a function of the magnetic gap-disc inertia and the applied magneto-motive force (mmf). Usually this delay is minimal and can be influenced by the circuit used to energize the clutch magnetizing coil.
The third delay is influenced by the L/R discharge circuit characteristics, but more important is the residual magnetism associated with all the ferromagnetic material due primarily to the magnetic hysteresis characteristics associated with the material.
The fourth delay is a function of the force available to separate the coupling disc members.
It is to be understood that if a demagnetized sample of ferromagnetic material is subjected to a steadily increasing value of H (Magnetic Field Intensity), a maximum value of H is reached where the material is essentially saturated. If the magnetic field intensity H is then decreased to zero, the flux drops down on a curve of B vs H (Magnetic Induction in gauss' vs Magnetic Field Intensity in oersteds) that is different than the curve it rises on. If the current is reversed, the same will occur in the negative direction. This results in two spaced apart S-shaped curves meeting at their positive and negative ends so as to form a loop which is referred to as a hysteresis loop.
A ferromagnetic substance is capable of having an infinity of different hysteresis loops depending only on the maximum value of H reached on the rising characteristics before H is decreased.
The maximum value of H and of the B-H hysteresis characteristics are defined by the power requirements of the system.
In any event, the delay associated with the residual magnetism is the most important delay. From observations of the hysteresis loop, the residual magnetism can be brought to zero quicker to release the coupling magnetism sooner if the power to the magnetic coil first reverses at the time decoupling is required.
Heretofore, various methods and apparatus have been proposed for accomplishing the reversing effect. Examples of such previous proposals for reversing the magnetic effect are disclosed in the following patents:
______________________________________ U.S. PAT. NO. PATENTEE ______________________________________ 2,615,945 Jaeschke 4,306,268 Cooper ______________________________________
The Jaeschke U.S. Pat. No. 2,615,945 discloses a system for demagnetizing a fluid magnetic gap material utilizing a mechanically operated switch to discharge a capacitator in the reverse direction of the original magnetization to decrease the release time.
More specifically, in this system a DC current is simultaneously applied to a capacitator and to a field coil of an electromagnetic coupling. At the same time as the field coil is being energized from the voltage source, a resistor network coupled in series between the voltage source and the field coil is sequentially switched from a high resistance to a low resistance. Then, when it is desired to decouple the electromagnetic coupling, the voltage source across the coil is disconnected from the field coil and the capacitator is connected in reverse polarity across the field coil in a closed loop circuit arrangement.
The Cooper U.S. Pat. No. 4,306,268 discloses a complex switching system for shortening the time delay of demagnetization of a large lifting magnet to provide better accuracy of drop upon release of the magnet.
More specifically, the system provides an electromagnet having a coil which is energized with voltage at one polarity for picking up magnetizable pieces of material. Then, when it is desired to repel or drop those pieces of material from the electromagnet, the coil of the electromagnet is first connected in a closed loop dissipating circuit having impedence means therein for dissipating stored energy, and after the voltage across the coil of the electromagnet drops to the potential of the voltage source, the voltage source is connected in reverse polarity across the coil of the electromagnet to assist in the rapid change of magnetic polarity of the electromagnet to repel magnetized particles that adhere thereto.
As will be described in greater detail hereinafter, the method and apparatus of the present invention differ from the previously proposed control circuits by providing an inductor(preferably realized by a brake magnetizing coil) that is continuously energized while the magnetic polarity of a clutch magnetizing coil is reversed. By using the inductor or brake magnetizing coil in the method and apparatus, the current through the clutch magnetizing coil is very accurately controlled to provide predictable operating and release times thereby substantially reducing the "per cycle" time of operation.